to gain weight

Hi there!  Welcome to MyDiet&Excercise!

This is a really interesting question and one that I'm sure many other people are asking.  The simple answer is, just as to lose weight you must burn more calories than you consume; to gain weight, you must consume more calories than you burn. It's as simple as that!

I would caution you though, just as with weight loss, make sure you are gaining weight in a healthy way.  Eat a healthy balanced diet full of fruits, vegetables, protien and carbs and continue to be active. 

Write back and tell us about your strategies and results!

Be Well!

SS

Here's a great sharepost that was written a few months ago on healthy weight gain.  Hope this helps!